Integrated circuit for controlling a laser diode

ABSTRACT

The invention relates to an integrated circuit for controlling a laser diode, comprising a signal input for receiving a data signal; a signal output for connection to the laser diode; a modulator for modulation of the data signal; a current infeed connected to the signal output for the supply of a bias current; a coupling capacitor between the output of the modulator and the signal output in order to form a high pass with the differential resistor of the laser diode. Preferably, the circuit also comprises an active compensation circuit with a low pass connected to the output of the modulator; and a circuit which is connected to the low pass for producing a signal which is inversely proportional to the low pass output voltage and which is added to the output signal or subtracted from the output signal. The low pass can be controlled by a circuit guiding the data signal.

The present invention relates to an integrated circuit for controlling alaser diode, an integrated circuit for differentially controlling alaser diode, and also to an integrated circuit for controlling a laserdiode array having a multiplicity of laser diodes.

In order to achieve power losses that are as small as possibleintegrated circuits (ICs) for controlling laser diodes are beingdeveloped for ever decreasing supply voltages. Breakdown criteria ofmany of the integrated components used constitute a further motivationfor smaller supply voltages.

The operating point of the laser diode to be controlled is set by athreshold current and a modulation current, the modulation currentrepresenting the datastream. The currents generate a voltage drop acrossthe laser diode. The voltage drop across the diode may be so large thatthe laser can no longer be driven directly by the laser driver IC onaccount of the limiting supply voltage.

If the voltage drop across the laser diode is so large that the lasercan no longer be driven directly by the laser driver IC on account ofthe limited supply voltage or the necessary voltage drops across thedriver transistors, then it is known to allow the modulation of thelaser diode to be effected by means of a coupling capacitance. Duringthe modulation and transmission of the data in the radio frequencyrange, however, losses and mismatches due to parasitic elements occur onthe coupling paths both from the modulation output of the laser driverIC to the coupling capacitor and from the coupling capacitor to thelaser diode coupling unit. These parasitic elements may be, inter alia,bonding wire inductances and pad capacitances.

The object of the present invention is to at least reduce thesedisadvantages. This object is achieved by means of the inventionspecified in the independent claims. Advantageous refinements of theinvention can be gathered from the subclaims.

The invention provides an integrated circuit for controlling a laserdiode, having: a signal input for receiving a data signal; a signaloutput for connection to the laser diode; a modulator for modulation thedata signal; a current infeed connected to the signal output and servingfor the supply of a bias current; and a coupling capacitor between theoutput of the modulator and the signal output, for forming a high-passfilter with the resistor of the laser diode connected to the signaloutput.

Integrating the coupling capacitance into the laser driver IC reducesthe number of problematic coupling paths from the modulation output ofthe laser driver IC to the laser diode coupling unit. The first couplingpath between the modulation output of the laser driver IC and theexternal coupling capacitance is thus obviated. This advantage can beutilized for a single and a differential control of the laser diode.

In one refinement, the integrated circuit has an active compensationcircuit, having a low-pass filter connected to the output of themodulator and a circuit connected to the low-pass filter and serving forgenerating a signal which is inversely proportional to the low-passfilter output voltage. This circuit is connected to the signal output insuch a way that the signal which is inversely proportional to thelow-pass filter output voltage is subtracted from the output signal. Thelow-pass filter may be controllable by a circuit carrying the datasignal.

In one refinement, the integrated circuit has an active compensationcircuit, having a low-pass filter connected to the output of themodulator and a circuit connected to the low-pass filter and serving forgenerating a signal which is proportional to the low-pass filter outputvoltage. This circuit is connected to the signal output in such a waythat the signal which is proportional to the low-pass filter outputvoltage is added to the output signal. The low-pass filter may becontrollable by a circuit carrying the data signal.

In another refinement, the integrated circuit comprises an activecompensation circuit having: a low-pass filter connected to the outputof the modulator, a voltage inverter connected to the low-pass filterand serving for inverting the voltage generated by the low-pass filterin accordance with the low-pass filter transfer function; a voltagefollower connected to the output of the voltage inverter; and aconverter between the output of the voltage follower and the signaloutput, for generating a current which is inversely proportional to thelow-pass filter output voltage.

This refinement has the following advantage: the coupling capacitanceforms a high-pass filter with the differential resistor of the laserdiode. In this case, time constants that are as large as possible arenecessary for a broadband data transmission. The lower bandwidthlimitation is determined by the definition of the time constant of thehigh-pass filter. Since the differential resistor of the laser diode ispredetermined, the value of the coupling capacitance must be chosen tobe correspondingly large. However, large capacitances in ICs take uplarge areas on ICs in accordance with the capacitance per unit area andthey have a harmful capacitance with respect to the substrate (ground).

The active compensation circuit now makes it possible to be able toreduce the integrated capacitance with the lower bandwidth limitationremaining the same. To put it another way, the lower bandwidthlimitation is extended with the integrated capacitance remaining thesame through the application of the compensation circuit.

The invention additionally provides an integrated circuit fordifferentially controlling a laser diode, having a signal input forreceiving a data signal; a first signal output for connection to theanode of the laser diode; a second signal output for connection to thecathode of the laser diode; a modulator for modulating the data signal;a bias current supply connected to the cathode; a reference-groundpotential connected to the anode; a first and a second couplingcapacitor between a first and second output, respectively of themodulator and the first and second signal output, respectively; and afirst and a second compensation circuit connected in parallel with thefirst and second coupling capacitors, respectively, the first and secondcompensation circuits in each case having a low-pass filter, which isconnected to the first and second output, respectively, of the modulatorand can be controlled by a circuit carrying the data signal; and acircuit connected to the low-pass filter and serving for generating asignal which is inversely proportional and proportional, respectively,to the low-pass filter output voltage, which signal is subtracted fromthe output signal and, respectively, added to the output signal.

Depending on the refinement the cathode is inductively isolated from thereference-ground potential, or internally coupled to thereference-ground potential. Equally, the anode may be inductivelyisolated from the bias current supply, or internally coupled to the biascurrent supply.

The invention furthermore provides an integrated circuit for controllinga laser diode array having a multiplicity of laser diodes, having; asupply voltage feed; one or a plurality of signal inputs for receivingdata signals; a multiplicity of signal outputs for connection to thelaser diodes, a signal output (in single or differential embodiment) ineach case being assigned to a laser diode; a multiplicity of modulationstages for modulating the data signals, a modulation stage in each casebeing assigned to a signal output; and a respective coupling capacitorbetween the output of one of the modulation stages and the assignedsignal output, for the formation of a high-pass filter together with thedifferential resistor of the assigned laser diode.

The use of integrated coupling capacitors is particularly advantageouswhen operating a laser diode array (arrangement of a plurality of laserdiodes with a mechanical interconnection) by means of a laser driver ICwith a plurality of modulation outputs. An enlarged printed circuitboard construction is necessary when using external couplingcapacitances in a known manner. The integrated coupling capacitance ofeach modulation stage makes this unnecessary, however.

It is thus possible for the modulation outputs of the laser driver ICsto be directly connected to the laser diode coupling unit or the laserdiode array, for example by means of bonding wires.

In one refinement, an active compensation circuit is connected inparallel with each coupling capacitor, having: a low-pass filterconnected to the output of the assigned modulation stage; and a circuitconnected to the low-pass filter and serving for generating a signalwhich is inversely proportional to the low-pass filter output voltage.This circuit is connected to the respective signal output in such a waythat the signal which is inversely proportional to the low-pass filteroutput voltage is subtracted from the respective output signal. Eachlow-pass filter may be controllable by a circuit carrying therespectively assigned data signal.

In another refinement, an active compensation circuit is connected inparallel with each coupling capacitor, having: a low-pass filterconnected to the output of the assigned modulation stage; and a circuitconnected to the low-pass filter and serving for generating a signalwhich is proportional to the low-pass filter output voltage. Thiscircuit is connected to the respective signal output in such a way thatthe signal which is proportional to the low-pass filter output voltageis added to the respective output signal. Each low-pass filter may becontrollable by a circuit carrying the respectively assigned datasignal.

In another refinement, an active compensation circuit is connected inparallel with each coupling capacitor, having: a low-pass filterconnected to the output of the assigned modulation stage; a voltageinverter connected to the low-pass filter and serving for inverting thevoltage generated by the low-pass filter in accordance with the low-passfilter transfer function; a voltage follower connected to the output ofthe voltage inverter; and a converter between the output of the voltagefollower and the assigned signal output, for generating a current whichis inversely proportional to the low-pass filter output voltage.

The integrated circuit according to the invention may be employed in alaser driver IC for a 10 Gbit/s data transmission, which drives a VCSEL(Vertical Cavity Surface Emitting Laser) and operates with a supplyvoltage of 3.3 V.

Furthermore, the integrated circuit according to the invention may beemployed in a 10 Gbit/s laser driver IC having a plurality of modulationoutputs, which drives a VCSEL array and operates with a supply voltageof 3.3 V. For comparison: known 10 Gbit/s laser driver ICs operate onlywith supply voltages of 5 V.

Exemplary embodiments of the integrated circuit according to theinvention are illustrated with reference to the figures, in which:

FIG. 1 shows a basic circuit diagram of an exemplary embodiment of theinvention;

FIG. 2 shows an example of the realization of an active compensationcircuit;

FIG. 3 shows an example of the realization of a modulation currentoutput driver;

FIG. 4 shows a circuit diagram according to a first coupling example oflaser driver output and laser diode;

FIG. 5 shows a circuit diagram according to a second coupling example oflaser driver output and laser diode; and

FIG. 6 shows a circuit diagram according to a third coupling example oflaser driver output and laser diode.

FIG. 1 shows a basic circuit diagram of an exemplary embodiment of theinvention, having an active compensation circuit for expanding the lowerbandwidth limitation of the high-pass filter. The high-pass filter atthe modulation current driver output is formed by the integratedcoupling capacitance C1 and the differential resistor R1 of the laserdiode. C1 and R1 form the time constant τ1=R1*C1.

The active compensation circuit illustrated in FIG. 1 comprises alow-pass filter, a voltage inverter, and a voltage follower withdownstream voltage-current conversion. The mechanism of action is asfollows: the low-pass filter, comprising R2 and C2, filters the highfrequencies out of the data signal in accordance with its time constantτ2=R2*C2. An output voltage is produced in accordance with the low-passfilter transfer function, which output voltage is inverted with the aidof the inverter. A current which is inversely proportional to thelow-pass filter output signal is generated from the inverter outputsignal by the voltage follower with downstream voltage-currentconversion at R3. This output current of the compensation circuit issubtracted from the output current flowing through C1. Given a suitablechoice of the magnitudes of R2, C2 and R3, the output current of thecompensation circuit compensates for the band-limiting influence of thehigh-pass filter (R1, C1) on the lower frequency components of the datasignal; i.e. the signals absent downstream of the capacitor C1(frequency components fg1<(2πτ1)) are added again by the activecompensation circuit.

FIG. 2 shows an example of the realization of the active compensationcircuit (here with a single-ended output driver) for expanding the lowerbandwidth limitation of the high-pass filter formed from C1 and R1. Thelow-pass filter comprises R2 and C2. The inverter is realized by thecommon emitter connection T2, R4 and R5. The voltage follower withdownstream voltage-current conversion comprises the elements T3, R6, I1,T1 and R3.

FIG. 3 shows an example of the realization of the modulation outputdriver (here as a differential circuit). T1 and T2 operate in commoncollector connection on the integrated coupling capacitances C1 and C2.The resistors R1 and R2 operate as attenuation resistors. L1 and L2serve to compensate for parasitic capacitances. The active compensationcircuits are connected in parallel with the integrated couplingcapacitances.

FIGS. 4 to 6 show the application of the active compensation circuit invarious coupling forms for the laser driver output and the laser diode.FIG. 4 shows a differential control of the laser diode with inductiveisolation from the bias current supply and the reference-groundpotential. FIG. 5 shows a differential control of the laser diode withan internal bias current supply and inductive isolation from thereference-ground potential. FIG. 6 likewise shows a differential controlof the laser diode with internal coupling to the bias current supply andto the reference-ground potential.

It should be noted that the invention is not restricted to the exemplaryembodiment described, but rather encompasses modifications within thescope of protection defined by the claims.

1. An integrated circuit for controlling a laser diode, having: a signalinput for receiving a data signal; a signal output for connection to thelaser diode; a modulator for modulation the data signal; a currentinfeed connected to the signal output and serving for the supply of abias current; and a coupling capacitor between the output of themodulator and the signal output, for forming a high-pass filter with thedifferential resistor of the laser diode connected to the signal output.2. The integrated circuit as claimed in claim 1, having an activecompensation circuit having: a low-pass filter connected to the outputof the modulator; and a circuit connected to the low-pass filter andserving for generating a signal which is inversely proportional to thelow-pass filter output voltage, which signal is subtracted from theoutput signal.
 3. The integrated circuit as claimed in claim 1, havingan active compensation circuit having: a low-pass filter connected tothe output of the modulator; and a circuit connected to the low-passfilter and serving for generating a signal which is proportional to thelow-pass filter output voltage, which signal is added to the outputsignal.
 4. The integrated circuit as claimed in claim 2 or 3, thelow-pass filter being controlled by a circuit carrying the data signal.5. The integrated circuit as claimed in claim 1, having an activecompensation circuit having: a low-pass filter connected to the outputof the modulator, a voltage inverter connected to the low-pass filterand serving for inverting the voltage generated by the low-pass filterin accordance with the low-pass filter transfer function; a voltagefollower connected to the output of the voltage inverter; and aconverter between the output of the voltage follower and the signaloutput, for generating a current which is inversely proportional to thelow-pass filter output voltage.
 6. The integrated circuit as claimed inone of the preceding claims, the supply voltage essentially being 3.3V.7. An integrated circuit for differentially controlling a laser diode,having a signal input for receiving a data signal; a first signal outputfor connection to the anode of the laser diode; a second signal outputfor connection to the cathode of the laser diode; a modulator formodulating the data signal; a bias current supply connected to thecathode; a reference-ground potential connected to the anode; a firstand a second coupling capacitor between a first and second output,respectively of the modulator and the first and second signal output,respectively; and a first and a second compensation circuit connected inparallel with the first and second coupling capacitors, respectively,the first and second compensation circuits in each case having alow-pass filter, which is connected to the first and second output,respectively, of the modulator and can be controlled by a circuitcarrying the data signal; and a circuit connected to the low-pass filterand serving for generating a signal which is inversely proportional andproportional, respectively, to the low-pass filter output voltage, whichsignal is subtracted from the output signal and, respectively, added tothe output signal.
 8. The integrated circuit as claimed in claim 7, thefirst and second compensation circuits in each case having a low-passfilter connected to the first and second output, respectively, of themodulator; a voltage inverter connected to the low-pass filter andserving for inverting the voltage generated by the low-pass filter inaccordance with the low-pass filter transfer function; a voltagefollower connected to the output of the voltage inverter; and aconverter between the output of the voltage follower and the first andsecond signal output, especially, for generating a current which isinversely proportional to the low-pass filter output voltage.
 9. Theintegrated circuit as claimed in claim 7 or 8, the cathode beinginductively isolated from the reference-ground potential.
 10. Theintegrated circuit as claimed in claim 7 or 8, the cathode and thereference-ground potential being internally coupled.
 11. The integratedcircuit as claimed in one of claims 7 to 10, the anode being inductivelyisolated from the bias current supply.
 12. The integrated circuit asclaimed in one of claims 7 to 10, the anode and the bias current supplybeing internally coupled.
 13. The integrated circuit as claimed in oneof the preceding claims, the laser diode being formed by a VCSEL.
 14. Anintegrated circuit for controlling a laser diode array having amultiplicity of laser diodes, having; a supply voltage feed; one or aplurality of signal inputs for receiving data signals; a multiplicity ofsignal outputs for connection to the laser diodes, a signal output ineach case being assigned to a laser diode; a multiplicity of modulationstages for modulating the data signals, a modulation stage in each casebeing assigned to a signal output; and a respective coupling capacitorbetween the output of one of the modulation stages and the assignedsignal output, for the formation of a high-pass filter together with thedifferential resistor of the assigned laser diode.
 15. The integratedcircuit as claimed in claim 14, an active compensation circuit beingconnected in parallel with each coupling capacitor, having: a low-passfilter connected to the output of the assigned modulation stage; and acircuit connected to the low-pass filter and serving for generating asignal which is inversely proportional to the low-pass filter andserving for generating a signal which is inversely proportional to thelow-pass filter output voltage, which signal is subtracted from theoutput signal.
 16. The integrated circuit as claimed in claim 14, anactive compensation circuit being connected in parallel with eachcoupling capacitor, having: a low-pass filter connected to the output ofthe assigned modulation stage; and a circuit connected to the low-passfilter and serving for generating a signal which is proportional to thelow-pass filter and serving for generating a signal which is inverselyproportional to the low-pass filter output voltage, which signal isadded to the output signal.
 17. The integrated circuit as claimed inclaim 15 or 16, the low-pass filter being controlled by a circuitcarrying the data signal.
 18. The integrated circuit as claimed in claim14, an active compensation circuit being connected in parallel with eachcoupling capacitor, having: a low-pass filter connected to the output ofthe assigned modulation stage; a voltage inverter connected to thelow-pass filter and serving for inverting the voltage generated by thelow-pass filter in accordance with the low-pass filter transferfunction; a voltage follower connected to the output of the voltageinverter; and a converter between the output of the voltage follower andthe assigned signal output, for generating a current which is inverselyproportional to the low-pass filter output voltage.
 19. The integratedcircuit as claimed in one of claims 14 to 18, the laser diodes beingformed by VCSELs.
 20. The integrated circuit as claimed in one of claims14 to 19, the supply voltage essentially being 3.3 V.